Given that all major caprine lineages emerged in the Late Miocene and contain at least one but usually several species from the eastern Himalayan region, their most likely place of origin is between today's Tibet and Mongolia or nearby. The mountain goat's ancestors thus probably crossed the Bering Strait after they split from their relatives, presumably before the Wisconsinian glaciation. Unfortunately, no Pliocene mountain goats have been identified yet; the known fossil record is fairly recent, entirely from North America, and barely differs from the living animals. In the Pleistocene, the small prehistoric mountain goat Oreamnos harringtoni lived in the southern Rocky Mountains. Ancient DNA studies suggest that this was the sister species of the living mountain goat, not its ancestor; consequently, the living species would also date back to the Pleistocene at least.

The mountain goat is the only living species in the genus Oreamnos. The name Oreamnos is derived from the Greek term oros (stem ore-) "mountain" (or, alternatively, oreas "mountain nymph") and the word amnos "lamb".

Both billy (male) and nanny (female)[2] mountain goats have beards, short tails, and long black horns, 15–28 cm (5.9–11.0 in) in length, which contain yearly growth rings. They are protected from the elements by their woolly white double coats. The fine, dense wool of their undercoats is covered by an outer layer of longer, hollow hairs. Mountain goats molt in spring by rubbing against rocks and trees, with the adult billies shedding their extra wool first and the pregnant nannies shedding last. Their coats help them to withstand winter temperatures as low as −50 °F (−46 °C) and winds of up to 160 kilometres per hour (99 mph).

Close-up of head

A billy stands about 1 m (3.3 ft) at the shoulder to the waist and can weigh considerably more than the nanny (around 30% more in some cases). Male goats also have longer horns and longer beards than females. Mountain goats can weigh between 45 and 140 kg (99 and 309 lb), and billies will often weigh less than 82 kg (181 lb). The head-and-body length can range from 120–179 cm (47–70 in), with a small tail adding 10–20 cm (3.9–7.9 in).[3][4][5]

The mountain goat's feet are well-suited for climbing steep, rocky slopes with pitches exceeding 60°, with inner pads that provide traction and cloven hooves that can spread apart. The tips of their feet have sharp dewclaws that keep them from slipping. They have powerful shoulder and neck muscles that help propel them up steep slopes.[6]

Mountain goats are the largest mammals found in their high-altitude habitats, which can exceed elevations of 13,000 feet (4,000 m). They sometimes descend to sea level in coastal areas although they are primarily an alpine and subalpine species. The animals usually stay above the tree line throughout the year but they will migrate seasonally to higher or lower elevations within that range. Winter migrations to low-elevation mineral licks often take them several kilometers through forested areas.[7]

Mountain goats are herbivores and spend most of their time grazing. Their diets include grasses, herbs, sedges, ferns, mosses, lichens, and twigs and leaves from the low-growing shrubs and conifers of their high-altitude habitat.

In the wild, mountain goats usually live 12 to 15 years, with their lifespans limited by the wearing down of their teeth. In zoos, however, they can live for 16 to 20 years.

Mountain goat kid at Cawridge, Alberta

Mountain goats reach sexual maturity at about 30 months.[8] Nannies in a herd undergo synchronized estrus in late October through early December, at which time males and females participate in a mating ritual. Mature billies stare at nannies for long periods, dig rutting pits, and fight each other in showy (though occasionally dangerous) scuffles. Young billies sometimes try to participate, but they are ignored by nannies; nannies also sometimes pursue inattentive billies. Both males and females usually mate with multiple individuals during breeding season, although some billies try to keep other males away from certain nannies. After the breeding season is over, males and females move away from each other, with the adult billies breaking up into small bands of two or three individuals. Nannies form loose-knit nursery groups of up to 50 animals.

Kids are born in the spring (late May or early June) after a six-month gestation period. Nannies give birth, usually to a single offspring, after moving to an isolated ledge; post partum, they lick the kid dry and ingest the placenta. Kids weigh a little over 3 kilograms (6.6 lb) at birth and begin to run and climb (or attempt to do so) within hours. Although they are mostly weaned within one month, kids follow their mothers closely for the first year of life (or until the nanny gives birth again, if this does not occur the next breeding season); nannies protect their young by leading them out of danger, standing over them when faced by predators, and positioning themselves below their kids on steep slopes to stop freefalls.

Nannies can be very competitive and protective of their space and food sources. They fight with one another for dominance in conflicts that can ultimately include all the nannies in the herd. In these battles, nannies circle each other with their heads lowered, displaying their horns. As with fights between billies during breeding season, these conflicts can occasionally lead to injury or death, but are usually harmless. To avoid fighting, an animal may show a posture of nonaggression by stretching low to the ground.

In regions below the tree line, nannies use their fighting abilities to protect themselves and their offspring from predators. Predators, including wolves, wolverines, lynxes, and bears, attack goats of most ages given the opportunity. The cougar is perhaps the primary predator, being powerful enough to overwhelm the largest adults and uniquely nimble enough to navigate the rocky ecosystem of the goats. Though their size protects them from most potential predators in higher altitudes, nannies must sometimes defend their young from golden eagles,[9] which can be a predatory threat to kids.[4] Nannies have even been observed trying to dominate the more passive, but often heavier bighorn sheep that share some of their territory.

Mountain goats can occasionally be aggressive towards humans, with at least one reported fatality resulting from an attack by a mountain goat.[10]